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A few days after injury, the axons in fruit flies disintegrate. However, in flies with mutations in
a gene called dSarm, axons can last as many as 50 days after injury.
Image courtesy of Freeman Lab / University of Massachusetts Medical School

Nerve Tonic

by Richard Saltus

Axon degeneration—an active process after injury—can be delayed.

A mouse with a random mutation changed forever the way scientists think about how injured nerves die—and how, conceivably, their death might be delayed or prevented.

When the mouse’s long nerve fibers called axons were cut off from the nerve’s cell body (home to the nucleus) or crushed, the nerves survived up to 10 times longer than in normal mice—weeks instead of days—before disintegrating. This fluke of nature, dubbed the WldS mouse when it was discovered 20 years ago, showed that damaged nerves don’t simply starve to death from lack of nutrients, and that rapid death is not inevitable.

“This was serendipity at its finest,” says neuroscientist Marc Freeman, an HHMI early career scientist at the University of Massachusetts Medical School.

Excited about treating neurodegenerative diseases, such as Huntington’s, Parkinson’s, and Alzheimer’s, researchers began an immediate search for the WldS mutation’s protective powers. “If these diseases are caused by inappropriately activated pathways that promote degeneration, you might be able to block the pathways,” explains Freeman.

New treatments may be a distant prospect, but Freeman and colleagues have provided fresh insight into the function of the protein that blocks the nerve degeneration in that mouse variant, and they’ve found a gene that, when knocked out, duplicates the protective effect in other mouse strains.

“This is the first gene that can be knocked out to obtain the [characteristics] of the WldS mouse, and might point us to good targets for therapy,” he says.

The WldS mouse put a new spin on Wallerian degeneration, a process described in 1850 by British scientist Augustus V. Waller (WldS stands for “slow Wallerian degeneration”). Waller noted that severed frog axons showed no change for 24 to 36 hours. Then, the part of the axon cut off from the cell body explosively disintegrated, leaving a trail of debris that was quickly gobbled up by macrophages and other immune cells.

“Waller concluded that the axons died from a passive wasting away because they were starved of nutrients from the cell body,” Freeman says. But the prolonged survival of WldS axons suggested otherwise. “We have been taught that axons are highly dependent on the cell body,” he says. “But now we know they can be far more autonomous than we give them credit for.”

Loss of support from the cell body, of course, does eventually spell the end for damaged axons. But scientists now believe—and Freeman has offered direct evidence—that axons actively initiate their own degeneration through a process not unlike apoptosis, or programmed cell death, that rids the body of damaged cells. The WldS protein puts the brakes on the process.

The Death Trigger

In the April 10, 2012, issue of Current Biology, Freeman described a molecular chain of events that helps trigger degeneration of injured nerve axons in Drosophila—fruit flies. A focal point of the action, says Freeman, is the mitochondria—energy-generating organelles that shuttle back and forth along the nerve axon like trucks on a highway.

In normal Drosophila, Freeman reported, cutting a nerve caused a sudden surge of calcium ions into the axon. The flood of calcium stopped the mitochondria in their tracks, causing a power shortage within the axon and triggering its rapid degeneration.

But in fruit flies with the WldS mutation the researchers found, the WldS protein stemmed the injury-induced elevation of calcium in the axon. In addition, mitochondria remained mobile and continued to power the nerve fiber.

Yet this revelation, and other studies of WldS, didn’t directly identify a native self-destruct pathway that Freeman was looking for. And it didn’t chart a course for potential therapies: Adding the WldS protein or its components to millions of nerve cells is not a practical option. He continued to hunt for a smoking gun—a gene or protein that triggered axon degeneration and which might be blocked or knocked out with drug treatment.

The Freeman team’s discovery of the dSarm/Sarm1 gene, reported July 27, 2012, in Science, filled the bill on both counts. Screening for mutations in fruit flies, the researchers isolated mutants whose axons were intact as much as 50 days after injury. The mutation affects a gene called dSarm (“d” for Drosophila) that is required for Wallerian degeneration: the mutant form suppresses the degeneration.

In mice, a similar gene, Sarm1, proved to operate in the same way, and provides the first direct evidence, says Freeman, “that axons actively promote their own destruction after injury. The Sarm1 protein is a member of an ancient axon death-signaling pathway.” Both expression of the WldS protein and loss of the Sarm1 protein dramatically slow degeneration, but whether through related or separate mechanisms remains unclear.

“There is growing evidence that the biochemical pathway that triggers the degeneration of axons in response to injury overlaps with the pathways of degeneration in chronic disease,” adds HHMI alumnus Marc Tessier-Lavigne, president of Rockefeller University and a coauthor on the paper. “We’re excited that the discovery of a new entry point into that pathway has the potential to help devise novel approaches to treating neurodegenerative disease.”

The researchers are moving toward experimental therapies. “We’re working with collaborators to see if knocking out the Sarm1 gene in animal models of diseases like Huntington’s could moderate the symptoms,” says Freeman. “Our hope is that the answer is yes.”